1. Field of the Invention
The present invention relates to the field of metal packaging, and, more particularly, to an improved method and apparatus for performing caring tests on drawn metal parts, such as cups or the like.
2. Description of the Prior Art
The longstanding use of tinplate has been directed in recent years to the manufacture of drawn parts used in the production of metal cans or the like. This process demands constantly increasing performance from the materials employed. The geometric quality of a can depends to a large extent on the anisotropy of the tinplate used. One method of determining the anisotropic characteristics of steel in a manufacturing process is known as the "cup earing" test. An initial step in the production of metal cans involves blanking out circular disks from a sample steel sheet and drawing the circular disks into flat bottom cups. Numerous devices are known for drawing disks into flat bottom cups, as evidenced by U.S. Pat. Nos. 4,414,836 and 3,494,169 to Saunders.
When a disk is drawn or pressed, the resulting cup exhibits a wavy top portion. In other words, the upper rim of the drawn cup includes alternating high and low portions thereon, hereinafter referred to as peaks and valleys, respectively. This phenomenon, known as "earing", occurs as a result of the orientation texture of the material. A typical cup of double reduced material will exhibit four peaks and four valleys. A cup made from isotropic steel would exhibit no earing.
In order to perform a cup earing test, the height of each peak and valley is typically measured by hand using a dial indicator. The foot of the dial indicator is placed on the cup rim, and the cup is rotated through 360 degrees. As the cup is rotated, the peaks and valleys are visually detected and the corresponding height readings are recorded. A "percent earing" is then calculated using the peak and valley height readings.
There are several problems and disadvantages with this method. First, the hand measurement of the cup is a time intensive operation. Second, the visual detection of the peaks and valleys is subjective, inexact and leads to poor repeatability of measurements. Third, the cup may exhibit a "wedge" condition due to certain drawing and anisotropic conditions, which condition can obscure peak and valley measurements. A wedge condition exists on the cup rim when a valley measurement is higher than an adjacent peak measurement. The presence of a wedge condition significantly increases the difficulty in locating the peak and valley positions for use in performing an accurate percent earing calculation.